A typical nuclear reactor installation, particularly of the pressurized-water coolant type, includes a reactor vessel containing an exposible core of fuel elements and positioned in a pit beside which there is a fuel element pit for fuel element decay or storage. The pits intercommunicate via a lock through which the elements can pass. Both pits are water flooded when the reactor's core of fuel elements is exposed so that the fuel elements may be removed and carried to the fuel element pit which contains a fuel element storage rack in which all of the fuel elements of the reactor's core can be retained with a physical arrangement preventing a condition of criticality. Normally, this fuel element pit is water flooded at all times.
After a reactor has been operating for what are normally fixed periods of operation, the fuel elements are burned-up or spent to different degrees. The fuel elements located in the central zone of the reactor wall are exposed to the strongest neutron flux and burn-up first and, therefore, are normally removed from the reactor core first and eliminated from the installation such as by being transported to a reprocessing facility. This means that replacement elements are required for the rector core's central zone.
For replacement elements those which surround the central zone are removed from their previously operating positions and inserted or shuffled into this central zone, providing these elements have claddings free from defects. By this zone-wise shuffling technique where the outer elements are shuffled to the central zone of the reactor core, the reactor is maintained in continuous operation excepting for the time required for the shuffling.
The prior art shuffling method, particularly in the case of pressurized-water coolant reactors, has consisted in removing all of the fuel elements from the reactor core and depositing them in the fuel element storage rack in the fuel element storage pit, these operations being conducted under water and by the usual remotely controlled element manipulating devices. Then each fuel element is removed from the rack and tested for cladding defects and returned to the rack, after which all of the fuel elements free from cladding defects are returned to the reactor pit for insertion in the reactor core in the position to which they should be shuffled. In other words, excepting for the burned-up central zone elements, the elements in the surrounding zone which are to be shuffled, are treated as a group of elements, the entire group being transferred to the storage rack, tested one after the other, sorted as required for the shuffling and returned in the form of a group to the reactor in the adjoining pit. All of this time the reactor installation must be in a shut-down condition and out of power production. This is an undesirably long time which in the case of a medium-sized plant, as exemplified by the fact that in the nuclear power plant at Obrigheim, some four to five days must be set aside for this zone-wise shuffling practice. This kind of a shuffling has been proposed in the German Patent No. 1,248,822.
Such prolonged shut-downs required for this prior art shuffling practice involves prolonged stoppage of power production and, therefore, very substantial financial losses for any nuclear power plant, and there has existed for some time an urgent need to shorten this time required for shuffling the core's fuel elements.